<title>Summing Amplifier</title>
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<param name=startCircuit value="amp-sum.txt">
<param name=startLabel   value="Summing Amplifier">
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<p>
This circuit outputs the (inverted) sum of the voltages of two input
signals.  In this case, the first input is a 200 Hz sine wave, and the
second input is a 20 Hz <a href="http://en.wikipedia.org/wiki/Square_wave">square wave</a>.
<p>
The <a
href="http://en.wikipedia.org/wiki/Operational_amplifier">op-amp</a>
attempts to keep its &ndash; input at the same voltage as the + input
(which is at ground).  So, both input signals are driving 1k
resistors whose other end is at ground.  The current across the third resistor
is the sum of the two currents,
so the voltage drop must be equal to the sum of the two voltage drops,
which is the (negative) sum of the input voltages.

<p>
Next: <a href="e-logconvert.html">Log Amplifier</a><br>
Previous: <a href="e-amp-diff.html">Differential Amplifier</a><br>
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